BIOMG 1350 Notes Week 6
BIOMG 1350 Notes Week 6 BIOMG 1350
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This 4 page Class Notes was uploaded by genehan on Saturday September 3, 2016. The Class Notes belongs to BIOMG 1350 at Cornell University taught by Garcia-Garcia, M; Huffaker, T in Fall 2015. Since its upload, it has received 6 views. For similar materials see Introductory Biology: Cell and Developmental Biology in Molecular Biology and Genetics at Cornell University.
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Date Created: 09/03/16
BIOMG 1350 Professor Bretscher & GarciaGarcia Spring 2016 Week 6: Lecture 1 of 2 Monday, Feb 29, 2016 Lecture Title: Protein Sorting Lecture Keywords: signal sequences, nuclear pores, nuclear transport receptors, nuclear localization signal NLS, cargo protein, mitochondrial transport, translocator, secretory pathway, signal recognition particle SRP, transmembrane protein, stop and start transfer sequences 1. Iclicker question: Which of the following statements is true? a. Transporters and channels move ions across membranes at similar rates. (False, channels move more ions than transporters.) b. Both transporters and channels can move ions against the membrane potential. (True, the Na-K ATPase is an antiport so Na+ moves against the membrane potential. For a channel, if the concentration gradient is larger than the membrane potential, ions can move against the membrane potential. c. The glucose uniport only transports glucose out of the cell. (True, it will transport glucose down its concentration gradient.) d. The uptake of glucose is driven by the K+ electrochemical gradient. (No, it is driven by the Na+ gradient.) e. Transporters and channels undergo a conformation change during ion movement. (False, true for transporters, not for channels.) 2. How do proteins know where to go? a. Signal sequences in the primary sequence are necessary and sufficient to direct a protein to its destination. i. An ER protein has an ER signal sequence whereas a cytosolic protein has no signal sequence. ii. In an ER protein with half the signal sequence removed, it does not make it to the ER, demonstrating that it is necessary to send it to the ER. iii. In a cytosolic protein with the ER signal sequence attached, this shows that it is sufficient to target a protein to the correct organelle. b. Signal sequences are “molecular ZIP codes” to direct proteins to the correct compartment. 3. Nuclear transport does not require translocation across a membrane. a. Traffic in and out goes through nuclear pores. b. There are unstructured protein loops (not alpha helices or beta strands) that do selective transport in and out of the nucleus. c. Small molecules and proteins can freely pass through the pores whereas larger proteins and complexes need a receptor system to be transported in and out of the nucleus. d. Nuclear transport receptors move molecules through pores by binding with a prospective nuclear protein with a nuclear localization signal NLS, transport it into the nucleus and releases the protein inside nucleus and the receptor goes out again to be re-used. e. A cargo protein destined for the nucleus has a NLS. i. It binds to the nuclear transport receptor, and passes through into the nucleus. ii. Ran GTP binds to the receptor and the cargo protein is released. iii. Ran GTP and the receptor return to cytosol, and GTP is hydrolyzed by a ran GAP in the cytosol so ran GDP dissociates and the receptor can be reused. f. Ran GEF is associated with the chromatin so that there is ran GTP in the nucleus. g. Ran GAP is associated with cytosolic fibrils of nuclear pore so ran GDP is in the cytosol. 4. Iclicker question – which of the following would occur if you depleted the cell of Ran GAP? a. Ran would accumulate in the nucleus bound to the nucleus transport receptor. (No, there would be high levels of Ran GTP exported into the cytosol.) b. Ran would accumulate in the cytosol bound to the nucleus transport receptor. (True) c. The nuclear transport receptor would accumulate in the nucleus and Ran in the cytosol. (False, the receptor and Ran would be associated in the cytosol.) d. The nuclear transport receptor would accumulate in the cytosol and Ran in the nucleus. (No, they would be associated in the cytosol.) e. None of the above. 5. Mitochondrial transport occurs across membranes. a. Protein in mitochondria are encoded both in nucleus and mitochondrial genome. b. The cytosol-synthesized protein has a necessary and sufficient signal sequence and binds to a receptor on surface of mitochondria. c. The protein translocator translocates the protein, unfolding it and the protein goes across both membranes simultaneously. d. It refolds inside the mitochondrion and then the signal sequence is cleaved to yield the mature protein. 6. The endoplasmic reticulum is the entry point for proteins destined for a variety of location. a. The secretory pathway involves the ER, to the Golgi apparatus, and then outside the cell. b. Proteins destined for the lysosome, endosome, plasma membrane, golgi apparatus are made in the ER. c. One third of the different proteins in a cell are targeted to the secretory pathway. 7. An ER signal sequence is recognized by a signal recognition particle SRP (a ribonuclear particle consisting of an RNA and 6 associated proteins). a. The signal sequence on the N terminus of a growing polypeptide chain is recognized by the SRP and binds it, which slows or stops translation. b. Then the SRP binds to the SRP receptor on the ER. c. SRP leaves to be recycled and the ribosome engages the translocation channel, which lets translation resume so the protein is translocated across the bilayer into the ER lumen. d. The mRNA is translated and the protein is translocated across the membrane. Translation and translocation are coupled. e. The interaction of SRP with the SRP receptor does 3 things: i. Brings the translation arrested ribosome complex to the ER ii. Directs the synthesis of the polypeptide to the translcation channel iii. Relieves the block in translation f. The signal sequence is cleaved off in the lumen by a signal peptidase on the luminal side. 8. A transmembrane protein is integrated into the ER membrane. a. In the primary sequence, there are hydrophobic stop and start transfer sequences that allow a transmembrane protein to be integrated into the ER membrane.
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